1. Field of the Invention
The present invention relates generally to digital subscriber line transceivers for communicating using telephone subscriber loops, and more particularly, to techniques for performing clock frequency synchronization (timing recovery) for asymmetric digital subscriber line transceivers under TCM-ISDN crosstalk.
2. Background of the Invention
FIG. 1 depicts in a block diagram a relationship between a single transmitter 102 (central office (CO)) and single receiver 104 (customer premises equipment (CPE)) that use digital subscriber line (DSL) communications over copper telephone wires 106. The wider bandwidth needed for DSL transmission generates crosstalk interference among copper wire pairs bundled in the same cable binder. The level of crosstalk varies for different cable structures and materials. Some countries such as Japan and Korea use telephone cables with a paper-based “pulp” insulator rather than the plastic insulated cables (PIC) used in the United States. These pulp cables have high level of crosstalk between different services over copper wires bundled in the same cable binder. ISDN service has been deployed widely over copper wires. Crosstalk caused by ISDN service is one of the major interferences to other newly deployed DSL services since portions of the transmission band for ISDN service overlap with portions of the transmission band for DSL services.
In countries such as Japan, where the noisy pulp cables are installed, a special TCM-ISDN system is deployed. This system is described in the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) specification G.961, Appendix III. The G.961 Appendix III system reduces crosstalk interference by switch synchronizing ISDN cards at the central office using Time Compression Multiplexing (TCM). TCM provides for ISDN signal transmission and reception during different time periods to reduce near-end crosstalk between ISDN services.
ITU-T ADSL standards G.992.1 and G.992.2 Annex C (hereafter “ADSL Annex C”) describe the operation of DSL modems under TCM-ISDN interference. Signal transmissions from DSL modems are switch synchronized to a 400 Hz TCM Timing Reference (TTR) generated at the central office. The TTR signal is the master clock signal for determining when the central office modem (the “CO 102 modem”) and the customer premises equipment (the “CPE 104 modem”) should transmit and receive ISDN and DSL signals.
Within the same cable binder, TCM generates a time varying noise environment. During the first half period of the TTR signal, the CO modem is dominated by near end crosstalk (NEXT) interference, and roughly speaking, during the second half period the CO modem is dominated by far end crosstalk (FEXT) interference. The reverse is true for the CPE 104 modem. FIG. 2 is a diagram illustrating the relationship between TTR, ISDN, and G.992.2 timing.
The TCM-ISDN crosstalk environment can be different depending on the length of the subscriber loop. On long subscriber loops, because the received signal is heavily attenuated, the NEXT interference is large compared to the received signal. The channel capacity in the NEXT period can be greatly reduced, sometimes be zero. On the other hand, in the FEXT period, the channel typically has good signal-to-noise ratio (SNR) because the FEXT interference is much weaker than NEXT, and small relative to the received signal.
FIG. 3 illustrates the relationship between the TTR signal, the ISDN NEXT/FEXT interference, and ADSL Annex C transmit frames. A “Sliding Window” operation specified in G.992.1 and G.992.2 Annex C defines the procedure for transmitting symbols under ISDN interference synchronized to the TTR signal. The FEXTR symbols are symbols completely inside the FEXTR period. The NEXTR symbols are symbols inside any portion of the NEXTR period. Thus, there are more NEXTR symbols than FEXTR symbols, as shown in FIG. 3. The CO modem 102 decides if a particular symbol is a FEXTR symbol or NEXTR symbol according to the sliding window and transmits the symbol according to bit maps corresponding to FEXTR and NEXTR symbols. Similarly, the CPE 104 modem decides if a particular symbol is a FEXTC symbol or NEXTC symbol and transmits the symbol according to bit maps corresponding to FEXTc and NEXTc symbols. The bit map for NEXT symbols can be all zero. In that case, only one bit map is used in each direction for FEXT symbols only. Although the exact symbol time is sliding relative to the TTR signal, the pattern is fixed by ADSL Annex C to be periodic with the period 345 symbols long, which is hereafter referred to as a “hyperframe.”
Referring to FIG. 4, there is shown the 345 training symbols that make up a hyperframe, and its relationship to the TTR signal including the mapping of NEXTR/FEXTR symbols. The only significant difference between the NEXTR and FEXTR symbols is the additive TCM-ISDN interference. Any symbol that is partially affected by NEXT interference is treated as NEXT symbol. The FEXT symbols represent a signal treated as transmitted entirely during the FEXT period. The remaining training symbols are treated as though they were transmitted during the NEXT period. From FIG. 4, it is observed that the TTR signal and the CO modem symbols are not aligned. However, over a period of 345 symbols, the TTR signal spans 32 or 34 periods, depending on the cyclic prefix selected by the CO modem. This least common multiple period is used by ADSL Annex C to define the hyperframe.
ADSL Annex C specifies a Discrete Multi-tone (DMT) system, which includes a plurality of tones having different carrier frequencies, each of which is modulated with different data. Tone 64 is used to transmit a “pilot tone” which enables synchronization of the clocks of the CO and CPE modems. The pilot tone is transmitted by the CO modem 102 (master) and synchronized to by the clock of the CPE modem 104 (slave). Using a conventional pilot tracking technique, the CPE modem checks the received pilot signal continuously to control the CPE modem clock. However, under the TCM-ISDN interference environment, use of a pilot tone transmitted during both NEXT and FEXT periods can lead to inaccurate synchronization of the clocks of the CO and CPE 104 modems because the pilot signal during NEXT period may be badly corrupted by the TCM-ISDN NEXT.
What is needed is a method and apparatus that improves the synchronization of the clocks of the CO and CPE under TCM-ISDN crosstalk.